Industrial fermentations predominately use glucose as a feedstock for the production of a multitude of proteins, enzymes, alcohols, and other chemical end products. Typically, glucose is the product of starch processing, which is conventionally a two-step, enzymatic process that catalyzes the breakdown of starch, involving liquefaction and saccharification. During liquefaction, insoluble granular starch is slurried in water, gelatinized with heat, and hydrolyzed by a thermostable alpha-amylase. During saccharification, the soluble dextrins produced in liquefaction are further hydrolyzed by glucoamylases, producing a high glucose syrup containing greater than 95% glucose.
Glucoamylases are exo-acting carbohydrases, capable of hydrolyzing both the linear and branched glucosidic linkages of starch (e.g., amylose and amylopectin) to produce fermentable sugars from starch (e.g., an enzyme-liquefied starch substrate). The fermentable sugars, e.g., low molecular weight sugars, such as glucose, may then be converted to fructose by other enzymes (e.g., glucose isomerases); crystallized; or used in fermentations to produce numerous end products (e.g., alcohols, monosodium glutamate, succinic acid, vitamins, amino acids, 1,3-propanediol, and lactic acid).
In view of the central role glucoamylases play in generating glucose from starch, it would be an advantage in the art to provide glucoamylase (GA) variants with improved properties for this conversion. Examples of improved properties of variant GA include, but are not limited to: solubility, hydrolytic activity, thermostability, pH activity, pH stability, reduced reversion product activity, and chemical stability.